1. Field of the Invention
This invention relates to a radiation detector for correcting detection signals of annihilation radiation-pairs. More particularly, this invention is directed to a radiation detector having a construction of converting radiation into fluorescence and determining the fluorescence, the radiation detector allowing to remove influences of afterglow of fluorescence by correction.
2. Description of the Related Art
Description will be given of a specific construction of a conventional positron emission tomography (PET) device for imaging distribution on radiopharmaceutical. The conventional PET device includes a radiation ring having radiation detectors arranged circularly for detecting radiation. The detector ring detects a pair of gamma-rays (an annihilation radiation-pair) having opposite directions to each other that is emitted from inside of a subject.
Next, description will be given of a construction of a radiation detector 51. As shown in FIG. 9, the radiation detector 51 includes a scintillator 52 having scintillation crystals arranged three-dimensionally, and a light detector 53 for detecting fluorescence from gamma-rays absorbed into the scintillator 52. The light detector 53 has a detection surface where multiple optical detecting elements are arranged in a matrix. The detection surface of the light detector 53 is optically connected with one surface of the scintillator 52. See Japanese Patent Publications No. 2009-222439 and No. 2009-229127.
Radiation entering into the scintillator 52 is converted into many photons to travel toward the light detector 53. Here, the photons, entering into the scintillator 52 while spatially spreading, enter into each detection surface of the light detector 53 arranged in a matrix. That is, many photons from fluorescence are simultaneously split into optical detecting elements for detection.
The radiation detector 51 determines a position in the scintillator 2 where fluorescence is emitted with use of detection data on fluorescence that is captured by two or more optical detecting elements. That is, the radiation detector 51 determines a position of a center of gravity in a luminous flux of fluorescence on the detection surface by two or more optical detecting elements. The position of the center of gravity means a position where fluorescence has been generated. Information on the position is used for identifying positions where radiopharmaceutical within the subject is accumulated.
However, the conventional detection of radiation noted above has a following drawback. Specifically, more doses of radiation entering into the radiation detector 51 may lead to incorrect identification of the position where fluorescence is generated.
This drawback concerns to a calculation process of the center of gravity in the luminous flux of fluorescence. Here, the calculation process is to be described. For simplification, it is assumed that the detection surface of the light detector 53 has 2 by 2 optical detecting elements, as illustrated in FIG. 10. The detection signals (correctly, an integrated value m where intensity data representing an intensity of fluorescence is integrated with a time since fluorescence is detected with temporal variations) of fluorescence outputted from the optical detecting elements a1 to a4 are denoted by A1 to A4, respectively. Here, the symbols A1 to A4 each indicate intensity of fluorescence detected by the optical detecting elements a1 to a4, respectively. A position X of the center of gravity in the luminous flux of fluorescence in an x-direction is expressed as follows under assumption of a center position as a starting point:X={(A1+A3)−(A2+A4)}/{(A1+A2+A3+A4)}  (1)
Specifically, more doses of radiation entering into the radiation detector 51 may lead to a phenomenon of apparently increased detection intensity of fluorescence. Next, description will be given of this phenomenon. FIG. 11 illustrates temporal variations in fluorescence that the optical detecting elements detect. Fluorescence emitted in the scintillator continues to be applied to the optical detecting elements for a while, although it is weak. Upon detecting of radiation, it takes much time to detect fluorescence in consideration of disappearance of such afterglow of fluorescence. Consequently, the radiation detector 51 determines fluorescence taking no account of the afterglow. Specifically, as illustrated in FIG. 11, the radiation detector 51 integrates detection intensity outputted by the optical detecting elements a1 to a4 during a period P with a time for calculating fluorescence detection intensity A1 to A4. Here, the afterglow is not considered as the fluorescence detection intensity. An event threshold value denoted by p1 in FIG. 11 is used for identifying fluorescence occurrence.
If more doses of radiation enter into the radiation detector 51, subsequent fluorescence is emitted before afterglow of previous fluorescence disappears. That is, fluorescence having a temporal width will temporally overlap each other. Specifically, as illustrated in FIG. 12, an afterglow component denoted by S is to be added in calculation of fluorescence detection intensity.
Such phenomenon occurs in every detection intensity of A1 to A4. Here, letting afterglow factor for A1 to A4 are denoted by α, β, γ, δ, respectively, a position X as the center of gravity calculated under existence of the afterglow factor is given as follows:X={(A1+α+A3+γ)−(A2+β+A4+δ)}/{(A1+α+A2+β+A3+γ+A4+δ)}  (2)The afterglow factor α, β, γ, δ each has an approximately equal value. Consequently, the afterglow factor in numerator of Equation 2 are offset. On the other hand, the afterglow factor in denominator of Equation 2 are not eliminated, but rather added to increase. Accordingly, the position X has a value in Equation 2 different from an actual value under influence of the afterglow factor. Specifically, existence of the afterglow factor may lead to increased denominator of Equation 2, thereby decreasing an absolute value of the position X. This phenomenon occurs in a y-direction orthogonal to the x-direction.
Description will be given of influences that the afterglow factor exert on mapping positions of the center of gravity. Suppose that fluorescence is emitted from each center of scintillation crystals that form the scintillator 2. Here, a point p in FIG. 13 is a fluorescence generating position. The radiation detector 51 identifies the fluorescence generating position, as illustrated in FIG. 13, under no influence of the afterglow factor.
Where fluorescence to be detected includes the afterglow component, the radiation detector 51 cannot correctly identify the fluorescence generating position illustrated in FIG. 13. That is, absolute values of X and Y in Equation 2 will be decreased apparently under the influence of the afterglow. Accordingly, as illustrated in FIG. 14, the fluorescence generating positions to be calculated deviate apparently toward the center of the scintillator 2, resulting in decreased distribution of the fluorescence generations. As above, with the conventional art, the afterglow of fluorescence causes incorrect identification of the fluorescence generating positions. Here, the position Y corresponds to a position of the fluorescence generating position in the y-direction.
This invention has been made regarding the state of the art noted above, and its object is to provide a radiation detector that allows correction so as to identify incident gamma-ray positions accurately with no influence of afterglow of fluorescence.